Hydrophobic soil – soil that is hydrophobic – causes water to collect on the soil surface rather than infiltrate into the ground. Wild fires generally cause soils to be hydrophobic temporarily, which increases water repellency, surface runoff and erosion in post-burn sites. Soil dispersion due to sodification causes similar problems.
Hydrophobic soils are created when hydrocarbon residue is created after organic material is burnt and soaks into empty pore spaces in the soils, making it impervious to water.
Dryness, plant chemicals, aromatic oils, and other chemicals also cause hydrophobicity.
References
Hydrophobic soil Wikipedia(Text) CC BY-SA